In traditional dentistry, the dentist makes a dental impression of the patient's teeth, when the patient needs a crown, a bridge, a denture, a removable, an orthodontic treatment etc. An impression is carried out by placing a viscous liquid material into the mouth, usually in a dental impression tray. The material, usually an alginate, then sets to become an elastic solid, and, when removed from the mouth, provides a detailed and stable reproduction of teeth. When the impression is made, cheek retractors are arranged in the patient's mouth to avoid that the soft movable cheeks affect the impression of the teeth.
Today direct 3D scanning of the patient's teeth can be obtained using an intraoral handheld 3D scanner instead of making a physical dental impression.
When scanning a rigid object in a location for obtaining a virtual 3D model of the rigid object, such as scanning teeth in the mouth of a patient by means of a handheld scanner, it may happen that movable objects such as the patient's cheeks, tongue, or the dentist's instruments or fingers are captured in the sub-scans, because these movable object are located for example between the surface of the teeth and the scanner, whereby the movable object obstruct the view of the teeth for the scanner. As the movable objects are movable they will typically move, and therefore it is likely that the movable object is only captured in one or a few subscans. Since a number of subscans are typically acquired for obtaining a virtual 3D model of, it is likely that there will also be acquired subscans of the same part of the rigid object but without the movable object obstructing the rigid object. Typically the movable objects will move or be moved very fast, since both the patient knows that his tongue should not touch or be near for the teeth when his teeth is scanned and the dentist knows that his instruments should not obstruct the visual access to the teeth. Therefore the movable object will typically only obstruct the visual access of the teeth for a very short time, and this means that the movable object will typically only be captured in one or few subscans. Furthermore, if the dentist notice that a movable object was present when he scanned a part of the teeth, he may return to scan the same part of the teeth where the movable object was before, and thus in most cases, there will also be subscans where the movable object is not present. The problem is then to differentiate between the surface of the movable object and the surface of the rigid object, such that only the surfaces originating from the rigid object are used when generating the virtual 3D model.
In prior art geometry and colour data are used to distinguish between a first and a second tissue, such as hard tissue as teeth and soft tissue as gums, tongue, cheeks, and lips.
EP1607041B discloses a method of providing data useful in procedures associated with the oral cavity characterized by comprising: providing at least two numerical entities (I1, I2, . . . , In), each said numerical entity representative of the three-dimensional surface geometry and colour of at least part of the intra-oral cavity wherein said numerical entity comprises surface geometry and colour data associated with said part of the intra-oral cavity; wherein at least a portion of said entities (I1, I2, . . . In) comprise overlapping spatial data, comprising:                (a) for each entity providing at least one sub entity (IS′1, IS′2, . . . IS′n) comprising a first tissue data set comprising surface geometry and colour data, wherein said colour data thereof is correlated with a colour representative of a first tissue; and        (b) stitching said first tissue data sets together based on registering portions of said data set comprising said overlapping spatial data (I1, I2, . . . In) and        manipulating said entity to provide desired data therefrom.        
Furthermore, in image processing a method called space carving is used for building up a 3D model.
The article “A Method for Registration of 3-D Shapes” by Besl and McKay, IEEE Transactions of Patten Analysis and Machine Intelligence, vol. 14, no. 2, February 1992 discloses a method for accurate and computationally efficient registration of 3-D shapes.
However, none of the prior art considers the case where some of the objects in the location are movable.
Thus it remains a problem to distinguish between movable objects and rigid objects, when both movable objects and rigid objects are present in a location, when scanning in the location for obtaining a virtual 3D model of the rigid object.